The present invention relates to a process for producing methacrylic acid by the vapor phase catalytic oxidation of methacrolein with the molecular oxygen or a molecular oxygen-containing gas. More particularly, it relates to a process for producing methacrylic acid in high yield from methacrolein by using a multi-elemental catalyst of complex oxide containing Mo, P, As, Cu, Cr and O if necessary one or more of Ce, Nd, W or Fe with or without a reducing organic agent.
Many patents and patent applications on various processes for producing unsaturated carboxylic acids such as acrylic acid or methacrylic acid by the vapor phase catalytic oxidation of the unsaturated aldehyde such as acrolein or methacrolein have been disclosed.
The process for producing acrylic acid by the oxidation of a acrolein have been practically worked in a large industrial size because of development of the catalytic systems having high catalytic activity.
However, in the process for producing methacrylic acid from methacrolein which is similar to the former, the results have been remarkably inferior to the results of the oxidation of acrolein.
The main reason is as follows. The resulting methacrylic acid is gradually oxidized to and easily decomposed into acetic acid, carbon monoxide or carbon dioxide etc. whereby a desired single flow yield of methacrylic acid (selectivity of methacrylic acid) in the industrial scale has not been obtained successfully. In general, it is not advantageous in industry and economy that the unreacted methacrolein is separated and recovered and the oxidation of methacrolein is repeated because of loss and polymerization in the recovery. Accordingly, it is important to use a catalytic system which imparts high conversion of methacrolein and high selectivity of the object methacrylic acid.
The catalytic activity (space time yield) and the catalytic durability (life) are important for the industrial operation. In the conventional processes for producing methacrylic acid, the catalytic activities are low, the reaction temperatures are remarkably high whereby remarkable decreases of the catalytic activity caused by the thermal deterioration and the desired life of the catalyst can not be obtained in the industrial operation. On the other hand, when the reaction temperature is lowered so as to prevent the decrease of the catalytic activity, a desired single flow yield of methacrylic acid can not be obtained because of low catalytic activity.
The inventors have studied to overcome these various problems and have studied various problems for developing the catalyst system which has high catalytic activity at low temperature and high selectivity and have attained the present invention.
The processes for producing methacrylic acid from methacrolein have been known. For example, it had been found that the catalysts having high catalytic properties developed for producing acrylic acid from acrolein were used for the process for oxidizing methacrolein (Mo-V-W-Cu-Cr type) in Japanese Patent Publication No. 11371/1974, etc. When these Mo-V type catalysts were used, the catalytic activity was usually too high to obtain a desired selectivity of methacrylic acid, disadvantageously.
It had been known that the Mo-P type catalysts imparting mild reactivity were used as the catalysts for oxidizing methacrolein in a vapor phase to produce methacrylic acid. With regard to the catalysts, there are many disclosures of the patent applications.
For example, in Japanese Patent Publication 19774/1972, there is the disclosure of the catalysts containing the main components of Mo and P with at least one of elements such as Cd, Tl, Pb, In, and Ge. In Japanese Patent Publication No. 24288/1950, there is the disclosure of the catalysts containing Mo, P, Tl, and at least one of elements such as Si, Al or Ti. In Japanese Unexamined Patent Publication No. 126616/1974, there is the disclosure of the catalysts containing Mo, P and V components; and in the Japanese Unexamined Patent Publication No. 96522/1975, there is the disclosure of the catalysts containing Mo, P, V, and at least one of Na, K, Rb or Cs. In USP 3,875,220, there is the disclosure of the catalysts containing the main catalysts of Mo, P and V or at least one of the elements such as Bi, As, B, Ce, Cr, Ag, Fe, W, Pb, Mn, Tl, Te, Ni, Nb, Sn and Cu.
However, when these known catalysts are used, the single flow yield of methacrylic acid is too low or the life of the catalyst or the space time yield are not satisfactory for the industrial operations.
On the other hand, in Japanese Patent Publication Nos. 10652/1974 and 3297/1975, there is the disclosure of the catalysts containing the main components of Mo and P with As.
When these catalysts are used, excellent selectivity can be obtained. However, the catalytic activity is too low and the life is not satisfactorily long. Accordingly, the catalysts containing the other metal element, or a mixture of the metal element and the alkali metal element together with said components had been known. For example, in Japanese Patent Publication No. 23014/1975, there is the disclosure of the catalysts containing Mo, P, As with at least one of V, Cu, Fe and Mn. In Japanese Unexamined Patent Publication No. 41811/1975, there is the disclosure of the catalysts containing Mo, P, As with at least one of V, W, Cu, Fe, Mn and Sn and at least one of Li, Na, K, Rb and Cs. In Japanese Unexamined Patent Publication No. 115414/1976, there is the disclosure of the catalysts containing Mo, P, As and Cu or V and at least one of Li, Na, K, Rb and Cs and at least one of Mg, Al, Si, Ca, Ti, Zr, Ag, Sb, Te, Ba and Ta. Even though some catalysts imparted relatively high single flow yield of methacrylic acid however, the catalytic activities at low temperature such as about 280.degree. C. were unsatisfactorily low.
The inventors have studied to add various other metal components to the system of Mo, P and As. As the results, it has been surprisingly found that the multi-elemental complex catalysts containing Mo, P and As with both of Cu and Cr or both of Cu and Cr and at least one of Ce, Ne, W and Fe impart remarkably high catalytic activity, and moreover, when an organic compound such as dibasic carboxylic acids, oxycarboxylic acids is added for the reduction during the preparation of the catalyst, the yield is increased and the satisfactory results can be obtained in the industrial operation.